Transcranial direct current stimulation (tDCS) is a widely-used tool to induce neuroplasticity and modulate cortical function by applying weak direct current over the scalp. In this review, we first introduce the underlying mechanism of action, the brief history from discovery to clinical scientific research, electrode positioning and montages, and parameter setup of tDCS. Then, we review tDCS application in clinical samples including people with drug addiction, major depression disorder, Alzheimer's disease, as well as in children. This review covers the typical characteristics and the underlying neural mechanisms of tDCS treatment in such studies. This is followed by a discussion of safety, especially when the current intensity is increased or the stimulation duration is prolonged. Given such concerns, we provide detailed suggestions regarding safety procedures for tDCS operation. Lastly, future research directions are discussed. They include foci on the development of multi-tech combination with tDCS such as with TMS and fMRI; long-term behavioral and morphological changes; possible applications in other research domains, and more animal research to deepen the understanding of the biological and physiological mechanisms of tDCS stimulation.

National Natural Science Foundation of China(31400959)
; 973 Program(2014CB846102)
; 111 Project(B07008)
; Open Research Fund of the National Key Laboratory of Cognitive Neuroscience and Learning(CNLZD1306)
; Fundamental Research Funds for Central Universities(SWU1509422
; Open Research Fund of the Key Laboratory of Mental Health, Institute of Psychology, Chinese Academy of Sciences(KLMH2015G01)
; Research Program Funds of the Collaborative Innovation Center of Assessment toward Basic Education Quality at Beijing Normal University(2016-06-014-BZK01
; 15XDSKD004)
; SCSM-2016A2-15003)